The invention relates to heat exchangers, and in particular to a unit for dissipation of heat from a processor or other integrated circuit. One or more heat pipe tubes for thermally conductive contact with the circuit package extends along an edge of an array of parallel air convection fins. The heat pipe tube(s) are tapered toward a plane of contact with the circuit package, preferably having a flattened oval cross section. This maximizes contact surface area on the heat pipe tubes and also provides a shape that structurally engages and supports the parallel fins on the heat pipe tubes. A clamping fixture arranged in the array of fins provides another structural member and a point for application of force to urge the heat pipe tubes into conductive contact with the circuit package.
Various electrical semiconductor devices, such as large scale integrated circuits, voltage regulators, current switching devices, high speed or high current circuits and other similar devices, generate heat that can be deleterious to their own operation and must be dissipated. If the ambient air adjacent to the circuit is at a lower temperature than the circuit device, some heat energy is dissipated by heating of the ambient air. The relatively hot circuit device heats the relatively cooler air that comes into contact with the circuit device. The heated air is circulated by convection and replaced by cooler air, thus moving heat energy away from its source.
The rate of heat energy transfer frequently must be sufficient to keep the heat source below some specified limiting temperature. A number of techniques are used to facilitate movement and dissipation of thermal energy from a heat source. Some techniques rely on the nature of the heat sink, which could comprise a solid, liquid or gaseous medium. Other techniques rely on unique structural attributes. It is possible to use different mediums together. It is possible to vary the structural attributes to meet various objectives. Such objectives include the heat transfer rate, the thermal inertia of the sink, its size and weight, manufacturing and material costs, etc.
For dissipation of heat energy into the air, for example, maximizing the surface area of air contact is a consideration, often leading to heat sink structures with thin metal plates or fins for thermal conduction. Consideration also must be given to how the heat is coupled into the fins, often leading to solid metal base plate blocks for contact with the heat source, the base plate block being cast integrally with fins. Structures can be provided to engage with the base plate block, such as clamps or springs and for mounting of a supplemental fan to force air over the fins.
One technique for moving heat energy, which technique is very apt for compact or portable devices having digital processors or the like that generate substantial resistive heating, is to employ a heat pipe configuration to move heat energy from point to point. In a heat pipe arrangement, a captive heat transfer fluid typically is provided in closed thermally conductive envelope. The fluid circulates in a manner whereby heat is taken up at a point that is in thermal contact with the heat source, and the heat is released at a point in thermal contact with a heat sink.
The thermal path advantageously employs a cycle of phase changes of the heat transfer medium. The heat transfer medium is brought in a liquid phase to an evaporator. Heat from the circuit or other heat source boils or vaporizes the heat transfer medium at the evaporator. The resulting gaseous phase diffuses through the envelope and encounters a condenser associated with a heat sink. At the condenser, the gaseous phase is cooled and condenses back to a liquid. A return flow path re-circulates the condensed liquid phase back to the evaporator, closing the loop. In a heat pipe, capillary flow through a wicking material can provide the return flow path. The typical return flow path in the case of a thermo-siphon is gravity driven. Each phase change stores or releases a quantity of heat energy due to the latent thermal energy involved in the phase change itself.
Phase change heat exchange circuits as described can operate with a very modest temperature difference between the source (evaporator) and the sink (condenser) while moving heat energy. Nevertheless, it is a typical attribute of most heat pipe designs that a discrete area of the conductive envelope functions as the evaporator, and a different area that is more or less remote from the evaporator functions as the condenser. If the structure of the envelope is thermally conductive and the condenser part is very close to the evaporator part, then heat energy coupled into the envelope at the evaporator tends to heat the condenser by conduction through the material of the envelope.
The ultimate object of a heat dissipation structure is to couple heat energy from the area of the evaporator to that of the condenser. The use of a heat pipe with a phase change medium has the further object of maintaining the evaporator and condenser respectively above and below the vaporization temperature of the medium. In U.S. Pat. No. 6,1,63,073xe2x80x94Patel, an integrated heat sink and heat pipe are provided. The heat sink has a cast base plate and vertically extending fins, the fins being cast integrally with thee base plate. The base plate has one or more elongated openings that extend along the bottom of the base plate, and either open downwardly toward the heat source or are just barely placed below the surface so as to minimize material between the opening and the heat source. Elongated heat pipes are disposed in the elongated openings, which: are exclusively within the thickness of the base plate.
The ""073 Patel patent explains that the area of the heat sink is much greater than the area of the heat source. This might suggest that the area in direct contact with the heat source functions as the evaporator, and areas that are remote from the heat source function as condensers. The patent teaches that this structure reduces thermal, gradients in the heat sink. If in an ideal case there is no thermal gradient across the heat sink, then at that area in contact with the heat source, the temperature of the heat sink would be as low as possible, providing good coupling of thermal energy into the heat sink. That ideal case, however, presumably would not rely on a phase change between an evaporator and a condenser.
The Patel patent teaches alternative structures for the heat pipes that are placed along the side of the base plate that is to contact the heat source. In the embodiments wherein the openings on the underside of the baseplate are channels opening at the surface, the heat pipes can be D-shaped rather than round in cross section, with the flat side facing the heat source. The channels are complementary, with U-shaped cross sections, providing for full surface contact.
In a different sort of finned heat pipe arrangement, for example as shown in U.S. Pat. No. 5,329,993xe2x80x94Ettehedieh, the base plate part of a finned structure carries an array of passage:s that function as an evaporator, and these passages are coupled to standing hollow columns that are coupled to the passages in the base plate and function as condensers. An array of parallel (horizontal) fins is coupled to the standing (vertical) columns. This arrangement has structural advantages, but is complex.
It would be advantageous if thermal efficiency, mechanical complexity and production ease: could be maximized in a finned heat pipe arrangement that is at the same time compact and inexpensive.
It is an object of the invention to provide a heat sink that does not rely on a base plate block to contribute to the structural or thermal attributes of the heat sink.
It is an object to structure the cross sectional shape of a heat pipe so as to structurally engage complementary openings in a plurality of fins in a stack.
It is a further object to provide reinforced openings in heat exchange fins, which openings are easily manufactured, for fixing the relative positions of heat sink parts.
It is another object to adapt a heat pipe structure such that the fins and/or the heat pipe elements carry all necessary mounting hardware and provide a rigid and lightweight structure that is substantially entirely optimized for heat transfer functions.
These and other objects are accomplished by a heat sink for integrated circuits, which is limited substantially to a stack of heat transfer fins on a heat pipe tube. The heat pipe tube has a flattened oval cross section and fits a complementary opening through the fins. A channel can be formed by aligned openings at the edge of the fins, exposing the heat pipe for direct contact with the heat generating circuit. The fins snap-fit with the tube and can have a collar to space the fins and/or extend the surface area of engagement. The air contact areas of the fins can be flat, or can comprise continuous folded or rolled form material wherein the variation from a flat shape provides greater total surface area per unit of outside dimensions (e.g., per unit of footprint area).
According to one aspect, the heat pipe can be snap fit in a channel running perpendicular to vertically oriented fins, along the bottom of the stack. In another embodiment, the upward legs of two U-shaped heat pipe tubes carry the stack. In that case, the bottoms of the U-shaped tubes are presented for contact with the circuit, under a stack of horizontal fins. A clamp urges the heat sink against the heat source.
The structure is inexpensively built from the minimum necessary parts and provides advantages including compact size, light weight, good thermal efficiency, low thermal inertia and low cost.